Motor control system



May 4, 1943. G. R. PuRlr-'oY MOTOR- CONTROL SYSTEM Filed may 1s, 1942 WITNESSES:

- lNvENToR George/Z?. Pz/f M@ ATTOR EY Patented May 4, 1943 MOTOR CONTROL SYSTEM George R. Purifoy, Pittsburgh, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh Pennsylvania Pa., a corporation of Application May 13, 1942, Serial No. 442,763

s claims.

My invention relates, generally, to motor control systems and, more particularly, to systems for controlling the operation of the propelling motors of electric vehicles.

An object of my invention, generally stated, is to provide a control system for electrically propelled vehicles which shall be simple and eicient in operation and which may be economically manufactured and installed.

A more specic object of my invention is to provide a simplified and improved control system of the type described in Patent No. 2,254,911, issued September 2, 1941, to L. G. Riley, in which a motor driven accelerator is utilized to control the acceleration and the deceleration of an electrically-propelled vehicle.

Another object of my invention is to provide a simplified control system suitable for application on cars which are connected together for multiple-unit operation.

A further object of my invention is to prevent jerking of the cars in a multiple-unit train when power is reapplied to the motors after a stop has been made.

Still another object of my invention is to accomplish the foregoing result without utilizing special relays.

Other objects of my invention Will be explained fully hereinafter or will be apparent to those skilled in the art.

In accordance with one embodiment of my invention, both the acceleration and the deceleration of an electrically-propelled vehicle are primarily controlled by a motor driven accelerator of the type described in Patent No. 1,991,229, issued February 12, 1935, to L. G. Riley. In general', the accelerator is controlled in the manner described in the aforementioned Patent No. 2,254,911. However, in the present system the amount of equipment required is materially reduced.

For a fuller understanding of the nature and objects of my invention, reference may be had to the following detailed description, taken in conjunction with the accompanying drawing, in which:

Figure l is a diagrammatic View of acontrol system embodying my invention; and

Fig. 2 is a chart showing the sequence of operation oi a portion of the apparatus illustrated in Fig. 1.

Referring to the drawing, a pair of motors I and II may be utilized for propelling a vehicle (not shown). The motor I0 is provided with an armature winding I2 and a series-field winding I3. Likewise, the motor I I is provided with 4an armature winding I4 and a series-held winding I5. A line switch LSI is provided for connecting the motors I0 and II to a trolley I6 which engages a power conductor I'I that may be energized from any suitable source of power such as a generating station (not shown).

The motors I0 and I I are connected in parallelcircuit relation during acceleration of the vehicle. They may also be connected for dynamic braking with the field winding I5 of motor II connected across the armature I2 of motor I0 and the eld winding I3 of motor I0 connected across the armature I4 of motor II, thereby permitting the current in the armature windings to reverse and cause the motors to act as generators and decelerate the vehicle. A pair of switches BI and B2 is provided for establishing the dynamic braking connections.

Both the' acceleration and the deceleration of the motors I0 and II are primarily controlled by a'motor driven accelerator A which is of the same general type as the one described in Patent No. 1,991,229 issued February 12, 1935 to L. G. Riley. The accelerator A comprises a circular bus I8 inside of which is disposed a plurality of contact ngers 2I to 49, inclusive, which are progressively forced against the bus I8 by a pair of revolving rollers I9 and 20.

The rollers I9 and 20 are driven by a pilot motor PM through a shaft 4I. The pilot motor PM is provided with an armature winding 44 and two eld windings 45 and 46, one for each direction. of rotation of the motor. The energy for operating thev pilot motor and the control apparatus may be supplied by a battery or other suitable source .of control energy.

A limit relay LR is provided for controlling the operation of the pilot motor PM during both accelerationand deceleration of the Vehicle. As shown, the relay LR is provided with several dif.. ferent actuating coils which function to operate the relay during coasting of the vehicle as Well as during acceleration and dynamic braking. Thus, a series coil 4l is connected in the motor circuit during both acceleration and deceleration of the vehicle. A spotting coil 49 is connected across a resistor 5I to be energized in accordance with the motor current during coasting of the vehicle, thereby providing a means of spotting the accelerator A. A variable rate coil 48 is energized during acceleration of the vehicle and an additional rate coil 52 is energized during coasting and dynamic braking for controlling the accelerating and braking rates. The relay LR is also provided with a tickler coil 53 which functions to cause a vibratory action of the relay in a manner well known in the art.

In order to provide for changing the setting of the relay LR, thereby governing the rate of acceleration of the vehicle by varying the speed of the pilot motor PM which, in turn, controls the rate at which resistance is shunted from the motor circuit to vary the motor current, variable resistors 54 and 55 are connected in the energizing circuit for the rate coil 48. The resistors 54 and 55 are varied by means of a master controller MC which may be divided into two sections, one designated AC, which is utilized during acceleration of the vehicle, and the other designated BC, which is utilized during dynamic braking of the vehicle. Since the controller is operated by the operator 0f the vehicle, the accelerating and braking rates may be varied by the operator as desired.

The accelerator A is provided with resistors 58 and 59 for controlling the current in the motors I0 and I I. The resistor 58 is divided into a number of subdivisions which are connected to the contact ngers ZI to 3U, inclusive, and the resistor 59 is divided into subdivisions which are connected to the contact ngers 3| to 48, inclusive. In the present system, the resistors 58 and 59 are connected in the motor circuit in series-circuit relation during both acceleration and dynamic braking, thereby making it unnecessary to change the resistor connections when transferring from motoring to braking operation.

As described in the aforementioned Patent No.

2,254,911, a resistor` 5I and an additional resistor 6I are connected in the motor circuit in parallel-circuit relation to the resistors 58 and 59 during the motoring operation., Since part `of the motor current is diverted through the resistors 5I and 6I, the heating eiect on the accelerator is reduced and also arcing of the contact ngers on the accelerator is reduced.

As shown, the accelerator A is provided With a drum switch 62 having a plurality of contact segments 63 to B'I, inclusive, and cooperating contact fingers which engage the contact segments as the accelerator is driven by the shaft 4 I. The reference numerals 2|' to 30 indicate the contact fingers over which `the roller I9 travels while the Contact segments are engaged by their respective Contact fingers. It will be seen that the roller 28 travels over fingers 3I to l0-while the yroller I9 travels over ngers 2I to 30. The function oi the different segments of the drum switch 62 will be explained more fully hereinafter.

In addition to the accelerator and the control switches previously mentioned, numerous other switches g are provided and perforrncertain switching operations. These include a switch LS2 for shunting a resistor 68 from the motor circuit, a switch MI for connecting the motors to the accelerator during acceleration of the vehicle, a switch G for connecting the accelerator resistors to ground during acceleration, a switch M2 for connecting the motors directly to ground through the switch G after the accelerator resistors have been shunted from the motor circuit by the accelerator rollers, a switch R for connecting the resistors 5l and SI in the motor i circuit during acceleration, as previously described and a eld shunting switch S having contact members SI and S2 for shunting the eld windings I3l and I5, respectively'.

Inorder to permit the present system to be utilized on cars which are operated in multiple unit trains and controlled from one control station at the head of the train, a braking relay BR is provided. The relay BR permits dynamic braking to be established simultaneously on all the cars of the train. The energization of the relay BR is controlled by the braking controller BC which may also be utilized to control the air brake system (not shown). Thus when the braking controller at the head of the train is operated, all of the relays BR throughout the train are energized to permit dynamic braking to be established on all the motors of the train. The controllers AC and BC are electrically interlocked to prevent improper operation of the equipment.

As fully described and claimed in my copending application Serial No. 442,764, filed May 13, 1942, interlocks are provided on the switch R for so controlling the operation of the pilot motor PM that the accelerator may be stopped and held on any position by operating the switchr R which is controlled by the accelerating controller AC. Thus, the controller AC is provided with a switching position and the variable rate positions.

When the controller is actuated to the switching position, the switches LSI, MI and G are closed to connect the motors to the power circuit in series-circuit relation with the resistors 5 8 and 59 of the accelerator A and also in seriescircuit relation with the resistor 63. At this time, the accelerator A will remain in its initial position. After the car has started moving and the rate of acceleration is to be increased, it is only necessary to actuate the controller to one of the rate positions, thereby closing the switch R to connect the resistors 5I and 6I in parallelcircuit relation to the motors and also closing the switch LS2 to shunt the resistor 68 from the motor circuit. The closing of the switch R causes the accelerator to rotate, as the pilot motor is energized through interlocks provided on the switch R, as will be described more fully hereinafter.

If it is desired to stop and hold the accelerator on any given position, the controller AC is returned to the switching position, thereby opening the switch R to deenergize the pilot motor, and stopping the accelerator at the desired position. The opening of the switch R also slightly increases the total resistance in the motor circuit to providea cushioning effect on the motors, thereby providing smoother operation of the vehicle.

Thus, by opening and closing the Switch R, the accelerator is caused either to progress or to hold. If automatic acceleration is desired, it is only necessary to actuate the controller to any desired rate position and the progression of the pilot motor will be under the control of the limit relay LR. The foregoing feature is particularly desirable when operating a vehicle through congested traflic areas which require that the acceleration of the vehicle be interrupted frequently. Furthermore, the slight increase in the resistance in the motor circuit caused by the opening of the switch R and the switch LS2, in the foregoing manner, prevents jerking or jumping'of the cars during acceleration as the progression of the accelerator is stopped at a desired runnin speed.

In order that the functioning of the foregoing apparatus may be more clearly understood, the operation of the system will now be described in more detail. vAs explained hereinbefore, power may be applied to the motors I0 and I I by actuating the controller AC to the switching position, thereby closing the switches LSI, MI, and G to connect the motors to the power source. The energizing circuit for the actuating coil of the switch LSI may be traced from positive through a contact segment 1I of the controller BC, conductor 12, a contact segment 13 of the controller AC, conductor 14, an interlock 15 on the switch BI, conductor 16, the actuating coil of the switch LSI, conductor 11, and the contact segment 64 of the drum switch 62, to ground. Following the closing of the switch LSI, a holding circuit for the coil of the switch is established through an interlock 18 on the switch LSI. The energizing circuit for the actuating coil of the switch MI extends from the conductor 16 through the actuating coil of the switch MI to ground and the energizing circuit for the coil of the switch G also extends from the conductor 16 through the coil of the switch G to ground.

The closing of the switch LSI, MI, and G connects the motors to the power source through a conductor BI, the switch LSI, the resistor 68,

conductor 82, the armature winding I2 and the series-held winding I3 of the motor I 0, conductor 83, the switch MIl, conductor 84, the resistor 59 of the accelerator A, the bus I8, the resistor 58, conductor 85, and the switch G to the ground'. The circuit through the motor II extends from the conductor 82 through the series-eld winding 'I5, conductor 86, the series coil 41 of the limit relay LR, conductor 81, the armature winding I4, "conductor 83, the switch MI, and thence through the' accelerator A and the switch G to ground through a circuit previously traced.

` As explained hereinbefore, the motors I0 and 'II operate at a relatively slow speed since all ofthe accelerator resistance remains connected in the motor circuit. If it is desired to increase the speed of the motors, the controller AC is vactuated to one of the rate positions, depending upon the rate of acceleration desired. When the controller AC is actuated to one of the rate positions, the switch R is closed to connect the resistors 5I and 6I in parallel-circuit relation to the resistors of the accelerator A, thereby reducing the duty imposed upon the accelerator and also lmaking it possible to connect the resistors 58 and 59 of the accelerator in series-circuit relation during motoring as well as during dynamicbraking. The energizing circuit for the actuating coil of the switch R may be traced from a contact segment 88 of the controller AC through conductor 89, an interlock BI on the switch LS I, conductor 92, and the actuating coil of the switch R t ground.

Following the closing of the switch R, th switch LS2 is closed to shunt the resistor 68 from the motor circuit. The energizing circuit for the actuating coil of the switch LSZ extends from the conductor 16 through an interlock 93 on the switch R, conductor 94, and the actuating coil of the switch LS2 to ground.

As explained hereinbefore, the closing oi the switch R also establishes a circuit for energizing the pilot motor PM to advance the accelerator A, the pilot motor being under the control of the limit relay LR, which, as explained hereinbefore, is responsive to the motor current. The energizing circuit for the pilot motor may be traced from positive through a resistor 95, a conductor 96, the tickler coil 53, contact members 91 and 98 of the relay LR, conductor 95, an.

interlock IOI of the switch G, conductor |02', an

interlock |03 on the switch R, conductor |04, an interlock I05 on the switch M2, conductor I06, the contact segment 66 of the drum switch 62, conductor |01, the on iield winding 45 and the armature winding 44 of the motor PM to ground.

As explained hereinbefore, the motor PM advances the accelerator A under the control of the limit relay LR unless the controller AC is returned to the switching position to deenergize the actuating coils of the switches R and LS2, thereby causing these switches to open. The opening of the switch R opens the interlock |03 carried by this switch, thereby deenergizing the pilot motor PM independently of any action of the limit relay LR. Accordingly, the pilot motor PM is stopped and the accelerator A will remain in the position in which it was at the time of the opening of the switch R until the controller AC is actuated to a rate position to reclose the switch R, at which time the accelerator will continue to advance under the control of the limit relay.

The opening of the switch R opens the parallel circuit through the resistors 5I and 6I, thereby slightly increasing the total resistance in the motor circuit. Likewise, the opening of the switch LS2 increases the resistance in the motor circuit by reinserting the resistor B8 in series with the motors I0 and II. When these switches are reclosed, the resistors 5I, 6I and 68 are reconnected in the motor circuit in the manner hereinbefore explained. In this manner, the vehicle may be held at any desired speed when operating through traic and a smoother operation of the vehicle is obtained by the cushioning action of the foregoing resistors in the motor circuit.

When the accelerator A nears the end of its travel in a forward direction, the switch M2 is closed to connect the motors I0 and II directly to the ground. The energizing circuit for the actuating coil of the switch M2 may be traced from the previously energized conductor 92 through an interlock |08 on the switch LS2,

" conductor 109, the actuating coil of the switch M2, conductor I II and the contact segment 63 of the drum switch 52 to ground. A holding circuit for the coil of the switch M2 is established through an interlock H2 carried by the switch.

The closing of the switch M2 connects the motors IIJ and I I directly to ground through the switch G, thereby permitting the accelerator A to be returned to its initial position in preparation for an establishment of the dynamic braking circuit for the motors. The closing of the switch M2 also opens its interlock |05 thereby interrupting the energizing circuit for the pilot motor PM, which operated the motor in the forward direction. Furthermore, the closing of the switch M2 energized the off eld winding 46 of the pilot motor, thereby causing it to operate in the reverse direction to return the accelerator A to its initial position. At this time the energizing circuit for the pilot motor may be traced from positive through the resistor 95, conductor S5, an interlock II3 on the switch M2. conductor II4, the contact segment S1 of the drum switch 52, conductor II 5, the off field winding 45 and the armature winding 44 of the motor PM to ground.

If it is desired to permit the vehicle to coast, the motors I0 and II may be disconnected from the power source by actuating the controller AC to the oil position, thereby deenergizing the actuating coils for the switches LSI, LS2, MI,

M2, R and G. When the controller AC is re' turned to the oi position, the switches Bl, B2 andS are closed to establish a dynamic braking circuit for the motors provided the accelerator A has returned to its initial position to insert the full amount of its resistance in the motor circuit.

In this manner, a srnall amount of current is permitted to circulate through the motors during coasting of the vehicle, as described in Patent 2,078,684, issued April 27, 1937, to L. G. Riley. However, the circulating current is of such a low value that it does not materially aiiect the coasting characteristics of the vehicle. The energizing circuit for the actuating coil of the switch Bi may be traced from positive through a contact segment H5 of the controller AC, conductor li, an interlock H3 on the switch LS|, conductor H9, the actuating coill of the switch Bl, conductor i2| and the contact segment 65 of the drum switch 52 to ground. The energizing circuit for the actuating coil of the switch B2 extends from the conductor ||9, through the actuating coil of the switch B2, conductor i2|, and the segment 65 to ground. A holding circuit for the switches B| and B2 is established through an interlock |22 carried by the switch Bi. The energizing circuit for the switch S extends from the conductor ||9 through an interlock |23 on the switch B2, contact members |24 of the relay BR, conductor |25 and the actuating coil of the switch S to ground.

AS explained hereinbeiore, the closing of the switch S shunts the eld windings |3 and |5 of the motors I0 and respectively, through field shunts |26 and |21 thereby reducing the el'd strength or the motors to reduce the current generated by the motors. During coasting, the generated current is maintained at a relatively low value by the action of the accelerator A which is under the control of the limit relay LR during coasting, the relay LR being operated by the spotting coil 49 which is connected across the resistor 5| which is in the motor circuit at this time. The spotting coil 49 is designed to operate the relay at a relatively low value of current. The energizing circuit for the spotting coil may be traced from one terminal of the resistor 5| through conductor |28, an interlock |29 on the switch G, conductor |3|, contact members |32 of the relay BR, conductor |33, the spotting coil 49 and conductors |34 and 84 to the other terminal of the resistor 5|.

Since the current generated the machines I Hland during coasting is proportional to the speed of the vehicle, this current may be utilized for spotting the accelerator A, that is matching the position of the accelerator with the speed of the vehicle, thereby insuring that the accelerator will be in the proper position for the utilization of dynamic braking to decelerate the Vcar. As previously explained, the accelerator is under the control of the limit relay LR during coasting. The Contact members of the limit relay are so connected in the circuit for the pilot motor that the motor may be operated in either direction depending upon the speed of the car.

Thus, with the contact members 91 and 98 closed the pilot motor advances the accelerator to ydecrease the resistance in the motor circuit until the point is reached at which the motor current is sucient to operate the relay LR to openthe contacts 91 and 93. Should the car decrease in speed, which would resultin` lower motor current, the contact members 9`| and 98 are closed to further advance the accelerator.

However, should the car increase in speed during coasting, resulting in a sufficient increase in the motor current to cause the contact member 9T to engage a contact member |35 of the relay LR, the pilot motor will be operated in the reverse direction to return the accelerator towards its initial position, thereby increasing the resistance in the motor circuit and matching the position of the accelerator with the speed of the car.l The circuit for forward operation of the pilot motor may be traced from positive through the resistor 95, conductor 9S, tickler coil 53, contact members 9'| and 98, conductor 99, an interlock |36 on the switch Bl, conductor |02, an interlock |31 on the switch M|, conductor |04, the interlock |05 on the switch M2, conductor |06, a segment 66 on the drum switch 62, conductor |01, the on eld winding 45 and the armature winding 44 to ground. The circuit for reverse operation extends from the contact member |35 of the relay LR through conductor ||4, the segment 61 of the drum 62, conductor 5, the on eld winding 45 and the armature winding 44 to ground.

With a View to simplifying the system and reducing the amount of equipment required, the switch M2 is utilized to permit operation of the accelerator during coasting and braking, but to prevent its advancement while the vehicle is standing still. When the accelerator nears the end of its travel in a forward direction during either coasting or braking, the switch M2 is closed and, as explained hereinbefore, the operation of the switch M2 opens its interlock |05 to interrupt the energizing circuit for the on eld of the pilot motor and closes vits interlock ||3 to establish an energizing circuit for the ofi field of the pilot motor, thereby returning the accelerator t0 its initial position. During coasting or braking, the energizing circuit for the actuating coil of the switch M2 is established from the previously energized conductor ||9 through an interlock |4| on the switch B2 and conductor |09 to the actuating coil of the switch M2 and thence through the contact segment 53 of the drum switch 02 to ground, as described hereinbefore.

Thus, the switch M2 is closed when the accelerator nears the end of its travel in a forward direction. If, for instance, the car has coasted to a stop after a light brake application, the accelerator roller'will progress through the operation of the limit relay to cause the' switch M2 to close in the manner hereinbefore described. The closing of the switch M2 causes the accelerator to be returned to the initial position and be held there for a reapplication of power. The same action takes place on all the cars of a multipleunit train, thereby insuring that all accelerators are in the initial position for a reapplication of power.

In the event that both the controller AC and the controller BC are actuated to the ofic position; at any time during the operation of theV vehicle, the pilot motor PM is operated in the reverse direction to return the accelerator to its initial position through interlocks provided on the switches LSI and Bl. The energizing circuit for the pilot motor may be traced from conductor 96, through an interlock |42 on the switch LSl, conductor |43", an interlock |44 on the switch Bl, conductor I4, and thence to the off eld winding of the pilot motor through a circuit previously traced.

vIn the event that dynamic braking is required to decelerate the vehicle, the controller BC is actuated to one of the braking positions, thereby energizing the relay BR to change the calibration of the rate coil 52 and the spotting coil 49. In this manner the limit relay LR. is recalibrated for proper operation during dynamic braking. The rate coil 48 is normally energized through either the resistor 54 or the resistor 55, depending upon whether the vehicle is accelerating or braking and the rate coil 52 is energized thro-ugh a resistor |45. The energizing circuit for the rate coil 52 extends from either the conductor 89 or the conductor through either interlock |54 or |55 on the switch R and conductor |56 to the resistor |45.

As explained hereinbefore, the contact members |32 of the relay BR shunt a resistor |46 from the energizing circuit for the spotting coil 49 of the relay LR. The operation of the relay BR opens the contact members |32 to insert the resistor |46 in the circuit for the spotting coil and closes Contact members |41 to shunt a portion of the resistor |45 from the circuit for the rate coil 52. The energizing circuit for the rate coil 48 may be traced from positive through either contact segment |48 and the resistor 54 to a conductor |49 or through contact segment |5| and the resistor 55 to the conductor |49, thence through the rate coil 48 to ground. The energizing circuit for the actuating coil of the relay BR may be traced from positive through a contact segment |52 of the controller BC, conductor |53, and the actuating coil of the relay BR to ground.

The operation of the relay BR also deenergizes the switch S, thereby removing the shunting circuits from the iield windings for the motors I and which increases the excitation of these motors and permits them to increase the braking current vgenerated and causes an increase in the braking effect on the vehicle.

As explained hereinbefore, the accelerator is advanced under the control of the limit relay during dynamic braking, thereby controlling the braking current by varying the resistance in the motor circuit. The switch M2 is closed in the event that the accelerator is fully advanced, thereby shunting the resistors 58 and 59 from the motor circuit during dynamic braking in a manner similar to that during acceleration of the vehicle.

From the foregoing description, it is apparent that I have provided a simplified and improved system for controlling vehicles which operate in multiple-unit trains. The present system prevents jerking of the cars of a train by insuring that all of the accelerators are returned to their initial position before power can be reapplied to the propelling motors. Furthermore, it will be understood that the present system may be readily applied to the control of a plurality of vehicles operating in multiple-unit trains, the equipment on each vehicle being a duplicate of that illustrated and described.

Since many modifications may be made in the apparatus and arrangement of parts without departing from the spirit of my invention, I do not Wish to be limited other than by the scope of the appended claims.

I claim as my invention:

1. In a motor control system, in combination, a motor for propelling a vehicle, a power conductor, switching means for connecting the motor to the power conductor, switching means for establishing dynamic braking connections for the motor, a variable resistance accelerator for controlling the motor current during acceleration and dynamic braking of the vehicle, means for driving the accelerator, additional switching means for shunting the accelerator from the motor circuit, and interlocking means on the additional switching means for controlling the operation of said driving means to return the accelerator to a predetermined position and prevent its advancement from said position under predetermined conditions.

2. In a motor control system, in combination, a motor for propelling a vehicle, a power conductor, switching means for connecting the motor to the power conductor, switching means for establishing dynamic braking connections for the motor, a Variabie resistance accelerator for controlling the motor current during acceleration and dynamic braking of the vehicle, means for driving the accelerator, additional switching means for shunting the accelerator from the motor circuit, and interlocking means on the additional switching means for controlling the operation of said driving means to return the accelerator to a predetermined position and' prevent its advancement from said position while the vehicle is standing still.

3. In a motor control system, in combination, a motor for propelling a vehicle, a power conductor, switching means for connecting the motor to the power conductor, switching means for establishing dynamic braking connections for the motor, interlocking means on said switching means, a variable resistance accelerator for controlling the motor current during acceleration and dynamic braking oi' the vehicle, means for driving the accelerator, additional switching means for shunting the accelerator from the motor circuit, and interlocking means on the additional switching means cooperating with the interlocking means on the switching means for controlling the operation. of said driving means to return the accelerator to a predetermined position and prevent its advancement from said position under predetermined conditions.

4. In a motor control system, in combination, a motor for propelling a vehicle, a power conductor, switching means for connecting the motor to the power conductor, switching means for establishing dynamic braking connections for the motor, interlocking means on said switching means, a variable resistance accelerator for controlling the motor current during acceleration and dynamic braking of the vehicle, means for driving the accelerator, additional switching means for shunting the accelerator from the motor circuit, and interlocking means on the additional switching means cooperating with the interlocking means on the switching means for controlling the operation of said driving means to return the accelerator to a predetermined position and prevent its advancement from said position while the vehicle is standing still.

5. In a motor control system, in combination, a motor for propelling a vehicle, a power conductor, switching means for connecting the motor to the power conductor and for establishing dynamic braking connections for the motor, a variable resistance accelerator for controlling the motor current during acceleration and dynamic braking of the vehicle, means for driving the accelerator, additional switching means for shunting the accelerator from the motor circuit, interlocking means on the switching means for conrolling the operation of said additional switching means, and interlocking means on the addi= tional switching means cooperating with tne in; terlocking means on the switching means for controlling the operation of said driving means to return the accelerator to a predetermined position and prevent its advancement from said position under predetermined conditions.

6. In a motor control system, in combinaticn, a motor for propelling a vehicle, a power conductor, switching means for connecting the motor to the power conductor and for establishing dynamic braking connections for the motor, a variable resistance accelerator for controlling the motor current during acceleration and dynamic braking of the vehicle, means for driving the accelerator, additional switching means for shunting the accelerator from the motor circuit, interlocking means on the switching means for controiling the operation of said additional switching means, and interlocking means on the additional switching means cooperating with the interlocking means on the switching means for controlling the operation of said driving means to return the accelerator to a predetermined position and prevent its advancement from said position while the vehicle is standing still.

7. In a motor control system, in combination, a motor for propelling a vehicle, a power conductor, switching means for connecting the motor tothe power conductor and for establishing dynamic braking connections for the motor, a master controller for controlling said switching means, a variable resistance accelerator for controlling the motor current during acceleration and dynamic braking of the Vehicle, means for driving the accelerator, additional switching means lfor shunting the accelerator from the mo# tor circuit, interlocking means on the switching means cooperating with the master controller for controlling the operation of said additional switching means, and interlocking means on the additional switching means cooperating with'the interlocking means on the switching means for controlling tl'ie operation of said driving means to return the accelerator to a predetermined position and prevent its advancement from said position under predetermined conditions 8. In a motor control system, in combination, fa motor for propelling a vehicle, a power lconductor, switching means for connecting the motor to the power conductor and `for establishing dynamic braking connections for the motor, a master controller for controlling said switching means, Aa variable resistance accelerator for controlling the mot'or current during acceleration and dynamic braking of the vehicle, means for driving the accelerator, additional switching means for shunting the accelerator from the motor circuit, interlocking means on the switching means cooperating with the master -controller yfor controlling the operation of said additional switching means, and interlocking 'means on the 'additional switching means cooperating with the interlocking means on the switching means for controlling the operation of said driving means to return the accelerator toa predetermined position and prevent its advancement from said position While the vehicle is standing still.

GEORGE R. PURIFOY. 

